I. Field of the Invention
The present invention relates in general to radiofrequency plasma spark plugs.
More specifically, the invention relates to a spark plug, known as a radiofrequency plasma spark plug, intended to equip a combustion chamber of an internal combustion engine, and comprising:                an annular shell of main axis D formed in a first conducting material and having first and second ends and an end circular surface with a main axis of symmetry D located at the first end of the shell;        a central electrode formed in a second conducting material extending along the main axis D and comprising an internal portion positioned inside said annular shell and an external portion positioned on the outside of said annular shell, nearer to the first end of the shell than to the second;        an electrically insulating component of annular shape extending at least around the internal portion of the central electrode such as to be inserted between the shell and the electrode, this insulating component covering only part of the external portion of the central electrode such that the uncovered part of the external portion is in contact with a gaseous mixture surrounding the spark plug.        
II. Description of Related Art
Ignition in gasoline internal combustion engines, which consists in initiating combustion of an air-fuel mixture in a combustion chamber of said engine, is relatively well controlled in current engines.
However, in order to comply with emissions standards, motor manufacturers have developed controlled-ignition engines capable of running on lean air-fuel mixtures, that is to say mixtures which contain an excess of air with respect to the amount of fuel injected.
Igniting a fuel-lean mixture is, however, difficult to control. As a result, and in order to improve the probability of successful ignition, it is necessary to have more fuel-rich mixtures around the spark plug at the instant the spark is produced.
Still with a view to increasing the probability that the spark plug will ignite the mixture, novel spark plugs with surface sparks have been developed in order to produce larger sparks to cope with the problem of the spatio-temporal meeting between the fuel mixture and the spark. Thus, a larger volume of mixture is ignited, and the probability of initiating combustion is therefore very greatly improved.
Such spark plugs are described in particular in patent applications FR97-14799, FR99-09473 and FR00-13821. Such spark plugs generate large-sized sparks from small potential differences.
Surface spark plugs have a dielectric (insulating component) separating the electrodes (one electrode being the annular shell and the other electrode being the central electrode) in the region where the distance between them is the smallest; the sparks formed between the electrodes are thus guided onto the surface of the dielectric. These spark plugs magnify the inter-electrode field at the surface of the dielectric. In order to do that, the elementary capacitors formed by the dielectric and an underlying electrode are progressively charged. The spark plugs generate a spark which travels along the surface of the insulator in the regions where the electric field in the air/gaseous mixture is the strongest.